Integrated circuit (IC) packages are assembled using lead frames. Each lead frame may include a plurality of leads, tie bars, and a die pad. The die pad is centrally located on the lead frame and provides a surface on which an IC die may be mounted. For power devices, the die pad serves as a heat sink wherein a lower surface of the die pad may be exposed to dissipate heat from the IC die.
There are different configurations of lead frames for the many different sizes of IC dies. A mono-thickness lead frame has the same thickness throughout. A dual-gauge lead frame uses materials having two different thicknesses. The thicker section becomes the die pad and the thinner section becomes the leads. A bi-component lead frame involves forming the lead frame and die pad separately, and then attaching the two together with a mechanical joint, such as with riveting or welding. The die pad is much thicker than the leads.
A thicker die pad is able to dissipate more heat away from the IC die. However, if a thickness of the die pad becomes too thick, it may be difficult to produce the end portions of the inner leads. A fine pitch is typically needed for the end portions of the inner leads, and this becomes difficult to produce with high precision when the die pad is much thicker than the leads.
One approach for a lead frame with a thicker die pad is disclosed in U.S. Pat. No. 6,501,156. The lead frame includes a die pad with a die pad main portion having a large thickness and a die pad peripheral portion having an intermediate thickness smaller than that of the die pad main portion. At least one support lead is connected to the die pad, and at least two first inner leads having a small thickness smaller than that of the die pad peripheral portion are arranged such that end portions thereof are opposed to the die pad peripheral portion.
Another approach for a lead frame with a thicker die pad is disclosed in U.S. patent application no. 2014/0191383. A packaged power device includes two lead frames. A first lead frame is a power frame with a thick die pad, and an IC die is attached to the thick die pad. The second lead frame has thin lead fingers, wherein a thickness of the second lead frame is less than a thickness of the power frame. One end of each of the thin lead fingers is attached to a bonding pad on an active surface of the power semiconductor die with a conductive adhesive. A molding compound covers the power frame, the lead frame, and the power semiconductor die.
Even in view of the above approaches for providing a lead frame with a thicker die pad, a problem with die pad delamination is still an issue. Delamination in IC packages is mainly caused by a coefficient of thermal expansion mismatch between the interfaces of two materials within the package. Die pad delamination is the separation between the IC die and the die pad on the lead frame. Die pad delamination reduces the total area of the IC die that is attached to the die pad, and is known to increase the thermal resistance of the IC package. This could lead to early thermal shutdown of a device which uses an exposed die pad to dissipate heat.